Conventionally, when digitized data is output by tracing a three-dimensional model surface, a straight line or the like set on the model surface is commanded as a tracing path and data is fetched at preset intervals while moving a sensor. When the model surface is not abruptly changed with respect to a direction in which the sensor is moved, the sensor fetches distance data from the measuring points on the model surface located substantially along the tracing path, but at a position where an inclination of the model is abruptly changed, the apparent measurement origin set to the sensor may bite into the model surface or leave therefrom.
As a result, a non-contact type optical sensor, for example, outputs measuring points deviating from a tracing point to a model. Therefore, a value obtained by adding the displacements of a measuring point to an actual position of a sensor at the time of measurement is output as digitized data.
When, however, a trace machining is actually executed based on digitized data output based on trace measuring points deviating from an original tracing path, a tool is finely oscillated with respect to a workpiece not only vertically but also horizontally about an initially set tracing path, which abruptly affects a machine tool for controlling a position of the tool. Further, a problem arises in that since the tool is controlled in a zigzag manner, the machining surface of the workpiece cannot be smoothly finished.